Conventional devices which automatically adjust the operating duration of a flash tube to obtain a predetermined quantity of emitted light employ a controllable switch (generally a quench tube of the gas discharge type), which is connected in shunt with the flash tube and triggered into conduction when the integrated light derived from the emission of the flash tube has reached a predetermined value. When the quench tube conducts, the exciting energy from flash capacitor bypasses the flash tube and is dissipated in the quench tube.
The portion of the available energy for the flash tube that is dissipated in the quench tube represents wasted energy, i.e. energy that does not result in useful light output. Also, the successive charging and recharging of the flash capacitor to and from a completely or nearly completely discharged state following each flash and quench operation limits the maximum repetitive speed of the device and increases its recycling time.
One manner of overcoming this disadvantage was proposed in the co-pending, co-assigned application Ser. No. 874,935 filed Nov. 7, 1969, now U.S. Pat. No. 3,612,947. A first thyristor gate, of the "interruptible" type that can be switched both on and off by pulses of respectively opposite polarity applied to a control electrode thereof, is interposed in the energizing path of the flash tube. The thyristor gate is opened to switch the flash tube on by a Schmitt trigger on similar bipolar threshold device which is excited by a reference voltage derived from an ignition pulse applied to a trigger electrode of the flash tube. A photosensitive element operatively associated with the light output of the flash tube generates a control signal that varies in proportion to the amount of light integrated. The Schmitt trigger is also coupled to the output of the integrating circuit in such a manner that when the control signal developed at the output of the integrating circuit has reached a predetermined value sufficient to overcome the previously established reference voltage at the input of the Schmitt trigger, the output polarity of the latter reverses to reclose the thyristor gate and terminate the flash.